Zeolites are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework type zeolites for which a structure has been established are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, C. H. Baerlocher, L. B. Mccusker and D. H. Olson, 6th Revised Edition, Elsevier, Amsterdam, 2007 and is also available at C. H. Baerlocher, L. B. Mccusker Database of Zeolite Structures: www.iza-online.org.
One known zeolite for which a structure has been established is the material designated as FER which is a crystalline aluminosilicate which consists of channels of 10-membered rings running parallel to the c-axis interconnected by channels of eight-membered rings running parallel to the b-axis and six-membered channels running parallel to the a-axis.
A number of zeolites having a FER framework type have been synthesised, including ferrierite and ZSM-35, for example U.S. Pat. No. 4,016,245 and U.S. Pat. No. 3,992,466. U.S. Pat. No. 4,016,245 describes a preparation for the zeolite ZSM-35 and its use in catalytic conversion of hydrocarbons. The zeolite has a composition expressed in terms of mole ratios of oxides (0.3-2.5)R2O:(0-0.8)M2O:Al2O3:>8 SiO2 wherein R is an organic nitrogen-containing cation and M is an alkali metal cation and is characterised by a specified X-ray powder diffraction pattern. U.S. Pat. No. 3,992,466 describes a process for converting hydrocarbons in the presence of a catalyst comprising a ZSM-35 crystalline aluminosilicate which serve to retard catalyst aging during the hydrocarbon conversion reaction.
Zeolites having the FER framework type have been found useful to catalyse the dehydration of methanol to dimethyl ether. The use of ferrierite in its hydrogen form to catalyse the dehydration of methanol is described, for example in the publications US 20090326281A, “Influence of catalytic functionalities of zeolites on product selectivities in methanol conversion” Seung-Chan Baek et al. Energy & Fuels, 2009, 23(2), pages 593-598 and “Determining an optimum catalyst for liquid-phase dehydration of methanol to dimethyl ether” Khandan, N et al. Applied Catalysis: General, vol. 349, Issues 1-2, 31 Oct. 2008, pages 6-12.
U.S. Pat. No. 6,740,783 describes an improved process for the preparation of dimethyl ether via the dehydration of a water-containing methanol feed in the presence of a zeolite catalyst in which zeolite the hydrogen cations are partially replaced with metal ions of Groups IA, IIA, IB and IIB of the Periodic Table or ammonium ions.
Korean patent application, KR 2009131560A describes the preparation of dimethyl ether by dehydrating methanol at 200-350° C. and 1-50 atmospheres pressure in the presence of a ferrierite based catalyst or a catalyst obtained by the partial introduction of alkali metal and/or alkaline earth metal ions.
U.S. Pat. No. 6,521,783 describes a process in which acetic acid, methyl acetate, methanol, dimethyl ether and water are fed to a hydrolysis/dehydration reactor which contains an ester hydrolysis catalyst and an alcohol dehydration catalyst which can be the same or different. The alcohol dehydration catalyst can be selected from a solid acid, heteropolyacids, acidic zeolites, titania or silica promoted alumina, aluminium phosphate or tungsten oxide supported on silica-alumina. The ester hydrolysis catalyst can be selected from acidic ion-exchange resins, acidic gamma alumina, fluorinated alumina, sulphate or tungstate promoted zirconia, titania or silica promoted alumina, aluminium phosphate, tungsten oxide supported on silica-alumina, clays, supported mineral acids, zeolites or heteropolyacids. In an example reported in this US patent the nature of the catalyst is not identified.
WO 2011027105 describes the production of acetic acid and dimethyl ether from methanol and methyl acetate at a temperature of 140 to 250° C. in the presence of a zeolite catalyst. The zeolite has a 2-dimensional channel system comprising at least one channel having a 10-membered ring. The zeolites identified as being of this type include ferrierite, ZSM-35 and clinoptilolite.
WO 9408920 describes a process for the highly selective skeletal isomerisation of linear olefin-containing organic feeds wherein linear olefins are contacted with a catalyst comprising ZSM-35, preferably microcrystalline ZSM-35 having its largest crystal dimension no greater than 0.5 microns, under isomerisation conditions to produce iso-olefins of corresponding carbon number.
Typically, zeolites, including those having a FER framework type, experience a decline in catalytic activity with the duration of their use which typically results in a loss of productivity to the desired products. This deactivation of the catalyst necessitates costly and time consuming regeneration processes to restore activity to the catalyst. Thus, means for extending the useful life of such zeolite catalysts is an on-going commercial objective. Consequently, it would be highly desirable to retard the aging of catalysts comprising zeolites having a FER framework type during their use in simultaneous dehydration-hydrolysis reactions of alcohols and esters, and in particular during their use in the conversion of methyl acetate and methanol by dehydration-hydrolysis to co-produce acetic acid and dimethyl ether.